(1) Field of the Invention
The present invention relates to a subscriber terminating apparatus and a packet processing method suitable for use in a DSU (Digital Service Unit) having, for example, a bridging function and a routing function.
(2) Description of Related Art
Because of recent spread of the Internet and development of Internet business, operators of personal computers and telephone subscribers (hereinafter referred as users) wish realization of high-speed, high-quality network environments.
The service in the access system for realizing this wish shifts from the ISDN (Integrated Services Digital Network) service that now spread to the xDSL (x Digital Subscriber Line) service. One of major reasons of this is that the xDSL service can use the metallic line that is presently used as an existing equipment as it is, and can realize a transmission rate not less than ten times that of ISDN.
xDSL signifies in general terms of ADSL (Asymmetric Digital Subscriber Line), MSDSL (Multi-rate Symmetric Digital Subscriber Line) and the like. ADSL is a high-speed transmission method in which data of 1.5 to 9.2 Mbps (megabit per second) is asymmetrically transmitted from the network to the user using a metallic line used for the subscriber telephone line. MSDSL is a symmetrical high-speed transmission system in which data can be bidirectionally transmitted/received.
When the user transmits or receives character codes such as an electronic mail, the user cannot clearly recognize the above transmission rate. When the user transmits or receives a file of a large capacity such as a still image or a moving image, the user can clearly recognize the difference. When the transmission rate is increased, the user comes to demand more improvement of the convenience and additional functions after enjoying pleasant net surfing.
One of the reasons to shift to the xDSL service is that competition to acquire subscribers among providers who provide Internet connection service to users (also referred as Internet service providers or carriers, hereinafter referred to as ISPs [Internet Service Providers]) becomes fierce. Namely, the number of ISPs tends to increase every year. Under such circumstances, each ISP comes to actively appeal its own feature, or there appears an ISP that is specialized to a specific field of business.
With increased ISPs having their own features, it is assumed that the user selects a desirable ISP in each specific field to connect to the Internet when subscribing for a connection service. Namely, the user changes an ISP to be connected according to each application, for example.
Here are defined terms used in the following description. IP (Internet protocol) datagram has an IP address that is an object of processing upper than Layer 3. An example of IP datagram shown in FIG. 15(a), for example, is composed of about 1500 bytes, which includes a transmission source IP address (transmission source network address) and a transmission destination IP address (transmission destination network address) each of 32 bits. An object of processing not upper than Layer 2 is referred to as a frame (for example, Ethernet frame). When there is no necessity to discriminate between IP datagram and the frame, the both are sometimes referred as packets. These terms will be used in the similar way unless specifically mentioned.
Further, a frame shown in FIG. 15(b) has a transmission source MAC (Media Access Control) address and a transmission source MAC address. The MAC address (physical address) is of 48 bits in many cases, which is recorded on a LAN card (not shown) to be connected to, for example, a personal computer when the LAN card is manufactured.
Bridging (bridge) is one of repeating functions of interconnecting networks, which is a function to identify a MAC address of a personal computer or the like in MAC layer at Layer 2 to repeat a frame.
In other words, the bridging function is a function of distributing frames at Layer 2 (Layer 2 level). A frame transmitted/received at Layer 2 cannot express an IP address, so as to be transmitted/received using only a MAC address (physical address). In concrete, a transmission source MAC address is not used, but only a transmission destination MAC address is used to be processed in all times. For this, a bridging apparatus (for example, a bridging modem) manages only transmission destination addresses, thereby realizing the bridging function.
Routing (IP routing) is a function of setting one route suitable for transfer among a plurality of routes leading to a final destination (destination) such as another network system or gateway server.
In other words, the routing function is mainly an IP datagram distributing function at Layer 3 (Layer 3 level). For this, a routing apparatus (routing modem) does not manage MAC addresses at Layer 2 like a bridging apparatus, but has a function equivalent to a general router to route only IP datagram.
FIG. 16 is a diagram illustrating general PPP (Point to Point Protocol) connection. An ATM (Asynchronous Transfer Mode) cell transmission system 50a shown in FIG. 16 is an ATM network system, which is a system that encapsulates IP datagram into an ATM cell and transmits the ATM cell, thereby transferring the IP datagram.
The ATM cell transmission system 50a shown in FIG. 16 comprises, for example, eight personal computers PC1–PC8, a DSU 51 having both of the bridging function and the routing function, an ATM network 2b to which ATM protocol is applied, and ISP-A to ISP-H of eight companies each having an ATM cell transmitting/receiving unit 32 and an authentication server 31. The user transmits IP datagram to the DSU (ADSL modem) 51 using a PPP connection, and transmits/receives IP datagram to/from an ISP. Whereby, an IP network is configured over the ATM network 2b. 
In the DSU 51, a protocol between the DSU 51 and the personal computer PC1 and a protocol between the DSU 51 and the ATM network 2b are converted (terminated). This termination is performed by changing or removing a header included in a packet.
Additionally, a direction from the ISP-A to the personal computer PC1 is referred as downstream (downward direction), whereas a direction from the personal computer PC1 to the ISP-A is referred as upstream (upward direction), occasionally, in the following description.
In order to realize bridging and routing, there are mainly two modes of the DSU 51 shown in FIG. 16. One mode is that the DSU separately has a bridging apparatus and a routing apparatus. The other is that blocks (processing blocks) realizing bridging and routing are integrally designed within the DSU 51, and the user operates an external change-over switch disposed in the DSU 51 to select either one of the functions. Incidentally, such modem is sometimes referred as an integrated modem.
In the latter embodiment, the user needs to reboot (restart) the DSU 51 after changing over the external switch in order to make a setting change effective, and the number of connectable ISPs is only one (for example, the authentication server 31 of ISP-D).
There have been proposed various techniques providing bridging and routing.
A technique disclosed in Japanese Patent Laid-Open Publication (Kokai) No. HEI 7-143178 (hereinafter, referred as publication 1) is aimed to readily control a repeating system or a repeater by sending data including a specific calling terminal address not corresponding to another terminal connected to the repeating system or the repeating apparatus from a terminal apparatus. The technique also provides a function of readily controlling the repeating system or the repeating apparatus without changing a protocol between the terminal apparatus, and the repeating system or the repeating apparatus.
According to the technique described in the publication 1 relating to known bridging and routing, when the transmission source and the transmission destination are in the same network, the routing function works. When the transmission source and the transmission destination are in different networks, the technique automatically discriminates it so that the routing function works to transfer data. Therefore, it is possible to connect both in the same network and between different networks.
Japanese Patent Laid-Open Publication (Kokai) No. HEI 7-221783 (hereinafter referred as publication 2) discloses a technique that can switch between the routing process and the bridging process to perform it according to a logical network number and a physical network number arbitrarily designated in each node when data is transmitted between nodes on different LANs (Local Area Networks).
A repeating system in the publication 2 is conscious of only IP layer to operate as a single path when LANs are connected.
When switching between bridging and routing in an integrated modem such as the above-described DSU 51, the user needs to manually change over the external switch or switch internal setting, or set using an external interface such as RS-232C. A reason why the external interface is necessary is that when the apparatus is used as a bridging apparatus, a control signal inputted from the outside cannot be expressed in Layer 2, thus the model cannot recognize the control signal. For this, an integrated modem can provide only either one of the both functions in view of full automation.
With respect to the full automation of the both functions, it is possible to switch between them on the ISP's side. Namely, the ISP allows the user to beforehand select and discriminate which function between routing and bridging is necessary, and the DSU 51 automatically switches, whereby the ISP provides different service modes to users.
Accordingly, the user does not need to set the switching by replacing the DSU 51 itself or by turning an external switch. The ISP can, on the other hand, improve quality of the service to the users.
However, the integrated modem executes bridging and routing using only a transmission destination address when transferring IP datagram over the ATM network 2b. For this, the integrated modem cannot recognize a transmission source of IP datagram. Thus, the integrated modem cannot distribute a VC (virtual Channel) number of an output destination in the apparatus using the IP datagram of the transmission destination, and cannot simultaneously connect to a plurality of ISPs.
As one type of general routing apparatuses (routers), there is a brouter apparatus (brouter) that can simultaneously execute bridging and routing. A function of the brouter is to route when a type of a upper protocol is one that the brouter can support, or bridge when a type of the upper protocol is one that the brouter cannot support, not determining with an address like the integrated modem. In this case, the user cannot simultaneously connect to a plurality of ISPs even with the brouter.
A demand for further improvement of quality of the ADSL service with commercially available DSLs is increased. It is forecasted that subscribers for the ADSL service will increase more and more in the future, and various demands will be made. The above problem is a part that particularly requires improvement.
In the technique described in the publication 1, the user does not switch between bridging and routing according to a different service mode provided by each ISP.
The repeating system described in the publication 2 relates only IP of a single path but does not relates ATM layer of the ADSL line when connecting LANs. For this, the repeating system cannot distribute a plurality of VCs in the modem.
The techniques described in the publication 1 and the publication 2 discriminate switching between bridging and routing using only an IP address (network number), but do not discriminate a personal computer that is a transmission source with a MAC address.